The present invention relates to a color image forming apparatus of the electro-photographic method such as a copying machine, printer, facsimile, etc., and in particular to a color image forming apparatus that utilize an intermediate image transfer belt.
Conventionally, a color image forming apparatus of the electro-photographic method utilizing an intermediate image transfer belt has been known in which the toner images of respective colors formed on the photoreceptors for different colors are transferred and superimposed onto the intermediate image transfer belt to form a color toner image, and the color toner image on the intermediate image transfer belt is transferred onto a transfer material such as transfer paper, etc. In other words, after the toner images that have been formed on the photoreceptors and that have been charged electrically with a specific polarity are transferred onto an intermediate image transfer belt by the use of electrostatic force, a color toner image on the intermediate image transfer belt is transferred onto a transfer material using electrostatic force.
Such a color image forming apparatus using an intermediate image transfer belt is used widely as a color image forming apparatus because the toner images formed on the photoreceptors for different colors are transferred successively onto an intermediate image transfer belt by superimposing one toner image on another, and further, because it is possible to transfer a color toner image superimposed on one another in one operation onto a transfer material.
In such a color image forming apparatus, even though the secondary transfer characteristics are good for the toner image of only single color, defects occur in the secondary transfer of multiple color toner images and hence it is difficult to obtain images of high quality. This is because the toner images formed on the intermediate image transfer belt have a wide range of adhered quantity from one layer up to a maximum of four layers, and hence the optimum conditions for secondary transfer according to the respective adhered quantities get shifted.
In view of this, methods have been proposed for improving the secondary transfer characteristics corresponding to the toner adhesion quantities of a wide range by providing a drive roller for driving the belt, providing an electric discharging unit before secondary transfer opposite to the drive roller and via the belt, and making uniform the potential of the color toner image on the intermediate image transfer belt (see, for example, Patent Document 1, Patent Document 2, and Patent Document 3).
Patent Document 1: Japanese Unexamined Patent Application Laid Open No. Hei 10-274892
Patent Document 2: Japanese Unexamined Patent Application Laid Open No. Hei 11-143255
Patent Document 3: Japanese Unexamined Patent Application Laid Open No. 2003-57959
However, although it is possible to improve to some extent the secondary transfer characteristics by making the potential of the toner image uniform, if the discharging unit is made to operate image force is generated between the intermediate image transfer belt and the drive roller there by making them get adhered to each other and the image quality decreases due to the load fluctuations generated by the drive of the belt.
In other words, although the slipping of the belt is suppressed while this adhesion force is operating thereby making it possible to drive the belt with a high accuracy, there was the problem that slight slipping of the intermediate image transfer belt is generated when the discharging unit is made ON/OFF during the image formation operations, thereby causing speed fluctuations due to load variations generated by the belt drive, and hence causing these to appear as image striations on the transfer material.